1. Field of the Invention
The present invention relates to an inner drum exposure apparatus (an inner surface scanning type light beam scanning exposure apparatus) for scanning a photosensitive surface arranged in an inner surface of a cylindrical drum by a scanning optical system of a light beam so as to executed an exposure process.
2. Description of the Related Art
Widely used is an inner drum exposure apparatus (an inner surface scanning type light beam scanning exposure apparatus) executing a scanning exposure process by introducing a light beam such as a laser beam or the like to a photosensitive surface of a recording medium arranged on an inner peripheral surface of a cylindrical drum by a light deflector. The recording medium on which an image is exposed and recorded is applied to an automatic developing machine as required, and a latent image formed on the recording medium is converted into a developed image. The inner drum exposure apparatus mentioned above is desired to be made of a multi-beam system for enabling a high speed exposure processing.
Conventionally, there is a technique of making the inner drum exposure apparatus in a multi-beam system with a method utilizing polarized light. The inner drum exposure apparatus of the method mentioned above combines respective light beams formed as two circular polarized lights in reverse rotating directions by using a light beam output portion in such a manner that respective optical axes coincide with each other, and outputs the combined light beam to a spinner side. The spinner changes a forward moving direction of the combined light beam to a direction toward an inner surface of the inner drum, by using a uniaxial crystal and a reflecting plate.
Further, there has been proposed a structure which scans a photosensitive recording paper on an inner surface of a drum so as to execute image recording, based on a plurality of light beams, by dividing a combined light beam into two light beams while changing an angle in correspondence to a rotating direction of circular polarized light, based on the function of a quarter wave plate and a uniaxial crystal (a Wollaston prism), and further integrally rotating the quarter wave plate, the uniaxial crystal and the reflecting plate serving as an optical element around a center axis by a main-scanning motor of a spinner or the like (for example, refer to Japanese Patent Application Laid-Open No. 5-27188)
Further, in an inner drum type multi-beam exposure system in the inner drum exposure apparatus, it is desirable to switch resolution of an image recorded on a recording medium. Accordingly, in order to structure the apparatus such that the resolution of the image can be switched, there can be considered a matter that a beam division width in a sub-scanning direction is made variable in correspondence to the resolution by rotationally adjusting the quarter wave plate and the uniaxial crystal (the Wollaston prism) installed on a rotation axis apparatus of the spinner so as to be integrally rotated around a direction of incidence of the light with respect to the rotation axis apparatus of the spinner.
However, if a mechanism for switching the resolution is installed on the rotation axis apparatus of a spinner required to rotate at an extremely high speed, such as some tens thousand revolutions per minute, the rotation axis apparatus of the spinner is enlarged in size and a weight thereof is increased. Accordingly, it is necessary to limit a rotational speed of the spinner rotation axis apparatus to a low speed, and there is generated a problem that a writing speed for scanning and exposing the photosensitive recording paper on the inner surface of the drum so as to record an image becomes slow.